1. Technical Field
The present invention relates generally to bipolar transistors, and more particularly, to a self-aligned raised extrinsic base bipolar transistor structure and method of fabricating the same.
2. Related Art
Self-aligned bipolar transistors with raised extrinsic base are the focus of integrated circuits fabricated for high performance mixed signal applications. Producing bipolar transistors for high speed applications requires improvements to the NPN junction to improve unit current gain frequency (fT) and maximum oscillation frequency (fMAX). fT is inversely proportional to base transit time (tb)(i.e., 1/tb) and collector-base capacitance (Ccb)(i.e., 1/Ccb). One approach to reduce transit time is to eliminate base widening due to thermal enhanced diffusion (TED) effects on the extrinsic base and loss of intrinsic base definition caused by the lateral diffusion of dopants during implantation of the extrinsic base. A deposited, raised extrinsic base eliminates implant damage in the intrinsic base region and therefore does not precipitate base widening during formation. A more important RF design parameter is fMAX, which is proportional to (fT/(Rb*Ccb))0.5. fMAX benefits from improved fT and collector-base capacitance (Ccb), but also requires reducing base resistance (Rb). There are several methods to improve Rb, an important aspect of which is emitter-base alignment. A fully self-aligned raised extrinsic base method will improve fT and fMAX of a bipolar transistor. Current approaches to achieve these improvements increase process complexity in order to maintain the extrinsic base self-aligned to the emitter, or employ a non-self aligned (NSA) structure in favor of a more simple process.
An approach for self-aligned with raised extrinsic base fabrication is disclosed by Chantre et al. in U.S. Pat. No. 6,472,262 B2. However, the Chantre et al. process results in less lateral control and higher base resistance due to continuous oxide layer 20, which leads to not only increased Rb but also poorer Rb control. Etch selectivity of silicon-germanium to silicon is required.
Another approach is disclosed in Ahlgren in US Publication No. 2003-0064555A1. However, this process is complex.
In view of the foregoing, there is a need in the art for a method of fabricating a self-aligned bipolar transistor structure that does not suffer from the problems of the related art.